Copper(II) cyclam-based complexes for radiopharmaceutical applications: synthesis and structural analysis

A novel side-bridged hybrid phosphonate/acetate pendant cyclam: synthesis, characterization, and 64Cu small animal PET imaging

Copper-64 (t(1/2)=12.7h; beta(+): 0.653 MeV, 17.4%; beta(-): 0.578 MeV, 39%) is produced in a biomedical cyclotron and has applications in both imaging and therapy. Macrocyclic chelators are widely used as bifunctional chelators to bind copper radionuclides to antibodies and peptides owing to their relatively high kinetic stability. A novel side-bridged cyclam featuring both pendant acetate and phosphonate groups was synthesized using a Kabachnik-Fields approach followed by hydrobromic acid deprotection. The Cu(II) complex of the novel ligand was synthesized, radiolabeling with (64)Cu was demonstrated, and in vitro (serum) stability was performed. In addition, in vivo distribution and clearance of the (64)Cu-labeled complex was visualized by positron emission tomography (PET) imaging. This novel chelate may be useful in (64)Cu-mediated diagnostic positron emission tomography (PET) imaging as well as targeted radiotherapeutic applications.

Cyclam derivatives containing three acetate pendant arms: synthesis, acid-base, metal complexation and structural studies

The ligands 1,4,8,11-tetraazacyclotetradecane-1,4,8-triacetic-11-methylphosphonic acid (H5te3a1p) and 1,4,8,11-tetraazacyclotetradecane-1,4,8-triacetic acid (H3te3a) were synthesized, the former one for the first time. The syntheses of these ligands were achieved from reactions on 1,4,8,11-tetraazacyclotetradecane-1,4,8-tris(carbamoylmethyl) hydroiodide (te3am x HI), and compounds (Hte3am)+, 1, and (H7te3a1p)2+, 4, were characterized by X-ray diffraction. Structures of two other compounds resulting from side-reactions, (H2te2lac)2+, 2, and (H4te2a2pOEt2)2+, 4, were also determined by X-ray diffraction. Potentiometric titrations of H5te3a1p and H3te3a were performed at 298.2 K and ionic strength 0.10 mol dm(-3) in NMe4NO3 to determine their protonation constants. 1H and 31P NMR titrations of H5te3a1p were carried out in order to determine the very high first protonation constant of this ligand and to elucidate the sequence of protonation. Potentiometric studies of the two ligands with Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Pb2+ metal ions performed in the same experimental conditions showed that the complexes of H5te3a1p present very high thermodynamic stability while complexes of H3te3a, particularly Co2+ and Zn2+, are even more stable. 31P NMR spectra of the cadmium(II) complex of H5te3a1p showed that the phosphonate moiety was coordinated to the metal ion. The UV-vis-NIR spectroscopic data and magnetic moment values of Co2+ and Ni2+ complexes of H5te3a1p and H3te3a together with the EPR of the corresponding Cu2+ complexes indicated that all these complexes adopt distorted octahedral coordination geometries in solution. This was confirmed by the single crystal structure of [Cu2(Hte3a)(H2O)3Cl]Cl0.5(ClO4)0.5 x 2H2O that showed two distorted octahedral copper centres bridged by a N-acetate pendant arm with a CuCu distance of 4.890(1) A. The first one is encapsulated into the macrocyclic cavity surrounded by four nitrogen and two oxygen donors from the macrocycle, whereas the second one is on the periphery of the macrocycle and is coordinated to two oxygen atoms of one acetate pendant arm in chelating fashion, one chloride and three water molecules.